gpio/mxc: make irqs work for fsl,imx21-gpio devices

The chained handler was set for the platform device with id == 0.
When the gpio devices are instantiated by a device tree, all have id ==
-1 and so the handler was unset resulting in unusable gpio irqs on
i.MX21 and i.MX27 (when using oftree).

Acked-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>